1. Field of the Invention
The present invention relates to improvements in or relating to transparency sensors.
It is well known to use sensors to detect the presence of paper in photocopying and/or printing apparatus. These sensors may operate in either a transmission or a reflection mode.
2. Description of the Prior Art
In the transmission mode, a radiation emitter, for example, a light emitting diode (LED), is positioned on one side of a paper path with an appropriate detector positioned on the other side of the paper path and aligned with the emitter to receive at least a portion of the radiation emitted therefrom. The detector, on detection of radiation from the emitter, produces an output signal indicating that no paper is present. When paper is in the paper path, radiation from the emitter reaching the detector is substantially reduced as the presence of paper cuts down the amount of radiation reaching the detector. This change in output signal produced by the detector indicates that paper is present.
In the reflection mode, the emitter directs radiation onto a surface, preferably a highly reflective surface, in the paper path and the detector is positioned to receive radiation reflected from that surface. When no paper is present, the detector provides an output signal and when paper is present, the output signal changes accordingly.
Moreover, it is desired to discriminate between sheets of paper and transparency sheets so that transparency sheets are not directed onto an unsuitable path in the apparatus, for example, to a duplex tray for double-sided copies. U.S. Pat. Nos. 4,540,887, 5,139,339, 5,329,338 and United Kingdom Patent Application GB 2 120 382 A describe optical sensing arrangements for effecting such discrimination and may be relevant to various aspects of the present invention.
U.S. Pat. No. 5,329,338 discloses an optical transparency detection and discrimination arrangement for use in an electronic reprographic printing system. The arrangement utilizes a plurality of diffuse reflective sensors positioned adjacent portions of the path over which a copy sheet moves. Each sensor is positioned so that its optical axis intersects with the copy sheet so that the angle of intersection lies within a specified range of angles for a maximum length of copy sheet. A pair of sensors are located at inlet baffles for detection and discrimination of the copy sheet. One sensor is arranged with its optical axis perpendicular to the copy sheet, and detects both paper and transparency copy sheets. The other sensor is arranged with its optical axis angled to the copy sheet so that only paper copy sheets are detected but not transparency copy sheets, the angle being chosen such that it is outside the range in which the sensor can detect a transparency, but within the range in which paper can be detected.
U.S. Pat. No. 4,540,887 discloses a high contrast ratio paper sensor arrangement which can be operated in both a transmissive and a reflective mode. In both modes of operation, a light emitter is arranged on one side of the paper path. A pair of photodetectors is used to detect radiation from the paper path. One detector is arranged on either the same side of the paper path as the emitter (reflective mode) to detect reflected radiation, or on the other side of the paper path to the emitter (transmissive mode) to detect transmitted radiation. In each case, the detector provides a first output signal in accordance with the radiation sensed either by reflection or transmission. The other detector is positioned on the same side of the paper path as the light emitter to detect diffuse radiation and provide a second output signal which corresponds to the diffuse radiation sensed. The two photodetectors are connected in a circuit wherein the second (or diffuse) output signal is subtracted from the first output signal.
U.S. Pat. No. 5,139,339 describes a media discriminating and media presence sensor in which a light emitter is arranged on one side of the media path. A pair of photodetectors is used to detect radiation from the media path. Both detectors are arranged on the same side of the media path as the emitter. The emitter and one detector (angle detector) are positioned at equal and opposite angles to the media surface while the other detector (normal detector) is positioned to be normal to the media surface. A non-reflective surface is used as a target when media is absent from the media path. When paper is present on the media path, the output signal from the angle detector is low and the output signal from the normal detector is high. When a transparency material is present, either alone or paper-backed, the output from the angle detector is high and the output from the normal detector is indeterminate and high respectively. When no media is present, the output signals from both detectors are low. The output signals from the respective angle and normal detectors provide a logic table indicating the presence of no media and the type of media.
GB 2 120 382 A describes an apparatus for detecting the presence of a transparency or thin paper using a light source and a light detector. Polarizing filters are placed in front of both the light source and detector. When the planes of polarization of both filters are parallel, attenuation resulting from the positioning of either a transparency or paper between the source and detector is significantly increased making possible the detection of a transparency or thin paper.
It is therefore an object of the present invention to provide a simple and effective way of detecting the presence of transparency sheets.